The invention relates to torque determination in AC motor drives in general, and more particularly in variable-frequency, variable-speed motor drive systems. The determination of the torque is important in AC motor drives, since the torque is an essential control parameter of an electrical motor drive. Constant torque motor drive operations under variable speed is one field of application of the invention. Another application is in motor drives operating at constant speed but variable torque. In the latter instance the function of the torque as a detected parameter is to allow detection of a torque limit characteristic of an overload requiring immediate corrective action on the motor drive. This is the case, for instance, where the AC motor drives a bottle conveyor. The excessive torque is indicative of a malfunction requiring to stop the conveyor.
In static-controller AC motor drives including a voltage-fed inverter for the generation of controlled AC current to the introduction motor, there is a DC link providing DC link current at the input terminals of the inverter. The product of the DC link voltage by the average of the DC link current is equal to the power delivered to the motor, that is: Power=Voltage.times.Current. On the other hand, Power=Speed.times.Torque, thus, EQU Torque=(Voltage.times.Current)/Speed
However, in an AC motor drive of the AVI type, i.e., DC link voltage adjusted motor drive, the DC voltage is automatically adjusted so that the voltage stays proportional to the frequency within the normal range of operation, thereby to keep the air-gap flux voltage drop constant. As a result, the torque will be proportional to the DC-link current in this type of motor drive. Sensing the DC-link current will be providing a signal representative of the torque. As an example of a GTO-controlled inverter associated with a three-phase thyristor-controlled AC/DC converter and operating as a DC/AC adjustable-frequency inverter, see D. A. Paice and K. E. Mattern "Application of Gate-Turn-Off Thyristors" in 1982 IEEE (IAS 82:27E) pp. 663-669.
This is no longer the case with a pulse-width modulated AC motor drive. Here, voltage control is performed by modulating the duty-cycle of the inverter-switches, i.e., changing the width of the pulses which determines the active periods of conduction of the switches within the duty-cycle. As an example of a pulse-width modulator for an inverter-fed drive system, see: B. K. Bose and H. A. Sutherland "A High-Performance Pulse-Width Modulator for an Inverter-Fed Drive System Using a Microcomputer" in 1982 IEEE (IAS 82:33A) pp. 847-853. The product of the DC-link voltage by the average DC-link current is still equal to the power being delivered to the motor, ignoring inverter losses. However, while the DC-link voltage is constant, the average DC-link current is here proportional to the motor torque only when the modulation index (i.e. the ratio of the actual motor voltage to the maximum voltage capability of the inverter) is ONE. In other words, the motor torque is proportional to the inverter input current divided by the modulation index. If information relative to the torque is to be derived from the information obtained with the inverter input current, this appears to involve a function which is rather complex and which must be expensive to implement.